A fuel injector includes a pressure vessel, a valve venting the pressure vessel, and a coil-driven magnetic circuit for driving the valve. The pressure vessel must not exhibit external fuel leaks during operation. Most fuel injector designs utilize multiple components that are welded together to create the pressure vessel.
Typically, fuel injector pressure vessel components are welded using a laser. A laser has been used successfully to weld joint configurations such as lap joints where overlapping surfaces of the components are jointed, butt joints where two components are joined end-to-end without overlap, and fillet joints in which material is removed on abutting parts to provide room for a weld bead. Lasers are suitable for welding small precision components together dependably and quickly in a production environment.
In many applications, the laser beam is held stationary as the part to be welded is moved or rotated to form the weld. To weld the hermetic pressure vessel used in fuel injectors, the beam is commonly held stationary while the part is rotated. For a hermetic weld of that type on tubular components such as those of the fuel injector pressure vessel, the “on” time for the laser beam is greater than the time it takes the part to make one revolution. The resulting overlap of the weld ensures that the weld is hermetic.
One common problem associated with such laser welding on tubular components occurs as the overlap of the weld is formed. Certain welding conditions and joint designs tend to result in a “blow out” of the weld bead, usually during final overlap of the weld. That “blow out” is created by rapidly increasing internal pressure on one side of the weld, due to a sudden rise in temperature related to the welding. The “blow out” occurs most commonly as the weld overlap occurs, although under certain conditions it is known to occur elsewhere. If an internal region to either side of the weld joint is undergoing a sufficient pressure increase, the weld “blow out” occurs when the molten weld pool is unable to resist the forces exerted by the pressure differential. The weld “blows out,” leaving a hole or gap in the weld bead. That hole typically leads to an increase in leak-related scrap during the assembly process.
For example, two components may be lap welded together at a continuous “interference fit” or press-fit region. Such welds have been known to exhibit “blow-out” regions at random locations relative as well as in the overlap. Those “blow-outs” are often at multiple radial locations throughout the weld. It has been theorized that in a press fit region, small cavities contain trapped air due to an imperfect surface finish of the components pressed together. When laser welding is attempted over those small cavities, the air inside undergoes a sudden change in temperature and expands. That expansion “blows out” the molten weld pool, leaving behind a void in the weld.
Alternatively, the two parts may be joined without a press fit and with clearance between the facing surfaces. No differential pressure is created, and therefore there are virtually no “blow-outs.” That joint design, however, has two significant drawbacks when used in a fuel injector application. First, any weld slag or oxides created by the welding process can escape from the weld joint into the valve body, creating internal contamination of the fuel injector. Such internal contamination in a precision device such as a fuel injector can have undesirable effects. Secondly, many designs require a press fit between the two components for processing reasons.
There is therefore presently a need to provide a method and system for reliably creating a hermetic weld joining tubular components of a fuel injector. To the inventors' knowledge, no such technique is currently available.